The recent explosion in wireless telecommunications has increased the demand for high performance millimeter wave radio frequency (RF) modules. One of the major cost and yield drivers for high frequency MMIC modules has been manual tuning to optimize module performance. The majority of MMIC RF amplifiers are not self biased. Therefore, each amplifier requires gate voltage (Vg) adjustment to tune the amplifier to its nominal operating conditions. This tuning normally occurs after the amplifiers have been assembled in the module and are connected to the power supply.
In order to have access to the chips in the module, probe stations are required. In addition, highly skilled operators are necessary to probe these small devices under a microscope. Damage to the chips is very common, even with veteran MMIC technicians. The needle-like probes used in the tuning cost thousands of dollars, and usually have a limited life because of wear and tear. It is estimated that it takes 20 to 30 minutes to probe each amplifier.
Many attempts have been made to automate the probing process, and there has been some limited success. The time and cost, however, involved in designing and using automatic module probing is extensive. In most cases, unique module designs prevent the use of a particular automatic probe station for more than a single module. These drawbacks have presented a challenge to many companies active in designing and manufacturing RF modules. As a result, high frequency modules are not produced in high volume. In most cases, manufacturers are forced to use expensive equipment and a large staff of qualified technicians to manufacture large numbers of RF modules.
Chip packaging for MMIC chips also is increasingly important. MMIC radio frequency modules have never been manufactured in high quantity amounts because the MMIC chips are fragile, typically 2 to about 4 mil thick, and difficult to handle. Air bridges, located over the surface of the chips, make it difficult to pick the chips from the top or exert pressure on the chips.
Special pick-up tools with pick-in-place equipment have been used to automatically pick-in-place the MMIC chips. These tools are expensive to manufacture and usually different MMIC chips require different tools. This has presented a challenge to different manufacturing companies because most automatic pick-in-place machines are limited to a limited number of tools for MMIC chips. In some cases, a manufacturer must use a series of different pick-in-place machines to assemble one radio frequency module. This is inefficient.
These MMIC radio frequency modules also are built in low volume amounts because there are usually a high number of MMIC chips, substrates and peripherals that are installed in each module. For example, a typical millimeter wave transceiver would have about 10 to about 15 MMIC chips, 15–20 pieces of substrate, and about 50–60 other peripheral components, such as resistors and capacitors. There is also a requirement that each of the components be connected via wire or ribbon bonds. This has also presented the challenge to millimeter wave module manufacturing companies.